Area Forecast Discussion
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Issued by NWS
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577 FXUS64 KLUB 191059 AFDLUB Area Forecast Discussion National Weather Service Lubbock TX 559 AM CDT Thu Sep 19 2024 ...New AVIATION... .SHORT TERM... (Today and tonight) Issued at 304 AM CDT Thu Sep 19 2024 Even with three days left until the first day of fall, our heatwave only ramps up today and may tie the daily record high at Childress. Temperatures at 1 AM in the upper 70s and low 80s were closer to our normal highs for the date thanks to muggy southeast winds supplying dewpoints in the upper 60s. This moisture will unfortunately mix out this afternoon as winds veer southwesterly in response to a surface low exiting eastern CO for the greener pastures of eastern KS. Add to this a marked increase in mid-level subsidence courtesy of a sprawling upper high centered near the Big Bend and the stage will be set for even hotter temps today, including a potential record high at Childress. Prospects for deep convection today are slim in light of the aforementioned negative factors, so will disregard the majority of CAMs that insist on storms along a very weak surface trough/dryline forecast to settle over our N-NW zones later today. Southerly breezes won`t be quite so muggy tonight, yet lows will still fare 8-12 degrees above normal. && .LONG TERM... (Friday through Wednesday) Issued at 304 AM CDT Thu Sep 19 2024 At the beginning of the period, the mid/upper-level pattern will feature an amplifying, southern-stream, positively-tilted trough digging into the Desert Southwest following the collapse of the synoptic-scale gyre previously anchored over the western U.S. days prior; and this trough will be accompanied by a 250 mb jet streak near 100 kt rounding its base and ejecting towards the Four Corners region on Friday. A coupling of the mid- and high-level flow should occur as a northern-stream trough noses into the Glacier Country, resulting in further amplification of the synoptic-scale flow and a substantial elongation of the 250 mb and 500 mb jet streaks. The subtropical ridge will also shift northward over southern Texas while slanting into a southwest-to-northeast-oriented manner via the influence of the upstream troughing. Farther east, the trough located across the eastern seaboard will be slow to progress but is forecast to gradually lose its amplitude even as a pseudo-Rex Block is restored over the northeastern U.S., maintaining a weakened blocking pattern across the Lower 48. Geopotential height tendencies will be neutral for most of the day Friday before becoming slightly negative towards the late-afternoon hours as the trough begins to attain its neutral-tilt over the northern Sonoran Desert. At the surface, a quasi-stationary front will be positioned along the lee of the southern Rocky Mountains, and bend northeastward towards the northern TX PH and OK PH where lee cyclogenesis should be underway. Leeward pressure falls will be moderated by the neutral geopotential height tendencies aloft; however, southerly winds will become breezy as the mass fields begin to respond to the amplified troughing to the west. The CWA will be enveloped within the moist sector, though deep boundary-layer mixing will temporarily delay the more-rapid surge of moisture return expected to occur the following day. Intense dry-bulbing will occur even as cirrus bands advect over the CWA from the southwest, with high temperatures similar to Thursday while remaining below record values. The subsidence layer in the mid-levels will also begin to erode as flow steadily backs to the southwest, but the magnitude and depth of dry air throughout the columns combined with the residual subsidence layer, in addition to the lack of low-level convergence, will result in a dry forecast on Friday afternoon and night. PoPs were removed from the western areas as the scattered convection in New Mexico will remain west of the TX/NM state line due to the orientation of the steering flow. By Saturday morning, the neutrally-tilted trough will continue pivoting over the Desert Southwest, with a well-defined, mid- and high-level closed low present as it emerges over the Four Corners region. NWP guidance is also indicating the 250 mb jet streak strengthening to near 110 kt as it ejects over central New Mexico on Saturday morning. The position of the 700 mb trough should also be located west of the TX/NM state line at this time, but WAA-induced showers and storms may advect into the far southwestern TX PH by daybreak Saturday. Rapid moisture return will occur during the overnight hours Friday into Saturday as leeward pressure falls intensify, with a plume of middle 60 degree dewpoints advecting poleward across the entire CWA by the mid-morning hours. Southerly winds will strengthen further compared to Friday, and winds were raised a couple of kt from the blended initialization as 998 mb surface low forms beneath the core of the jet streaks aloft. The closed low embedded within the trough should rotate over the Four Corners by Saturday afternoon, and the superposition of the mid- and high-level jet streaks will result in the CWA being beneath the right-entrance region as the afternoon progresses on Saturday. The initiation of isolated storms across the open moist sector will be possible, but most of the convection should be focused along the incoming cold front from the west as it overtakes the dryline. PoPs have once again increased slightly from the previous forecast assessments, with the highest coverage of storms delineated across the far southwestern TX PH and northwestern South Plains before storms becoming more-scattered with southward extent (as indicated in prior prognostications). A squall line will be possible, but the potential exists for updrafts to be sheared apart with southward extent across the CWA given the magnitude of cloud-layer flow despite favorable low-level storm-relative inflow facilitating wide updrafts. Severe-caliber wind gusts will be possible, especially if a squall line forms, given Inverted-V boundary-layer profiles and sufficient loading of hydrometeors. The risk for large hail should be tempered due to warm-cloud depths near 14 kft as the steepest geopotential height falls/mid-level cooling remains displaced to the north of the CWA, but a rogue nickel-to-quarter size hail event cannot be ruled out. The highest storm chances are forecast during the afternoon and into the late-evening hours across the Caprock Escarpment, and timing will continue to be refined in forthcoming cycles. The arrival of a polar cold front is also forecast to occur during the early-morning hours Sunday, and its southward movement may be reinforced by outflow from convection across the west-central High Plains, which would result in a faster progression than what is currently indicated in the official forecast. The low PoPs for Sunday were trimmed slightly from the blended initialization and eliminated after midnight CDT as the front should clear the CWA by then, with the best chances for storms across the Rolling Plains. The first day of fall will be accompanied by much cooler temperatures, and highs were nudged a couple of degrees lower with a 30-percent weighting of the 00Z statistical guidance applied to the NBM as the thinking continues to trend towards a faster frontal progression. Benign and mild weather is forecast for Monday as a weak surface high settles into the region, with renewed chances for storms returning by the middle of the week as global NWP guidances in agreement with at least broadly cyclonic flow encompassing the central U.S., but there remains significant bifurcation with respect to the amplitude of the troughing and whether or not it remains open. Therefore, the low, blended PoPs were maintained through the end of the forecast period as predictability remains limited. Sincavage && .AVIATION... (12Z TAFS) Issued at 559 AM CDT Thu Sep 19 2024 Continued VFR with light winds through tonight. && .LUB WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...93 LONG TERM....09 AVIATION...93